A web-offset printing press includes an inking assembly for each color of ink used in the printing process. Each inking assembly includes an ink reservoir and a plurality of hard nylon keys or a segmented blade disposed along the outer surface of an ink fountain roller. The amount of ink supplied to a roller train of the press and ultimately to a substrate, such as a web of paper, is adjusted by changing the spacing between the edge of the blade segments or the nylon keys and the outer surface of the ink fountain roller. The position of each blade segment or each key relative to the ink fountain roller is independently adjustable via an ink control system to thereby control the amount of ink fed to a corresponding longitudinal strip or ink key zone of the substrate.
Typically, ink is spread laterally from one longitudinal zone to adjacent zones due to the movement of vibrator rollers, which oscillate in a lateral direction relative to the substrate. The amount of ink on the ink fountain roller itself is also adjustable by changing the angle through which the ink fountain roller rotates each stroke. This generally occurs by adjusting a ratchet assembly, as is known in the art.
While such a printing press is running, a camera is typically used to continually monitor the printed output and to make appropriate ink key adjustments in order to achieve appropriate quality control of the color of the printed image. Specifically, the camera moves across the web to collect images of color patches on the moving web. Each pixel of the color patch images is then processed, and assigned a color value. Each color value is compared against a desired color value. If the absolute difference between the desired color value and the determined color value is outside some predetermined tolerance, the ink key is then controllably adjusted, thereby effecting a change in the ink flow rate.
It is not uncommon for printed images on the web, color patches in particular, to be corrupted by some printing artifact such as the effect of a paper fiber on the blanket roller (commonly known as a hickey), a droplet of ink, an indentation on the blanket, a slime hole in the paper, a scratch on the plate, or some other such defect. In this case, the measured color values of a defective color patch may not accurately reflect the color within the printed work itself. While methods for detecting a small defect in a color patch exist in marked color control systems, they are generally limited to eliminating small defects that do not encompass a relatively large portion of the color patch. Furthermore, these color control systems use techniques that assume that the color properties of the printed work remain constant over a defined area. However, the color properties of the print work may not remain constant. As a result, other techniques are needed to detect defects.
Color control systems for printing presses not requiring the use of color patches, or markless color control systems have been developed. Such markless color control systems measure color values in the printed work itself. Since the color of the printed work is measured directly in the markless systems, the correspondence between color patches and the work is not in question. However, these systems do not detect defects on the printed work. Even though the marked color control systems are configured to detect defects in the printed work, these defect detection techniques are applied to marked color control systems only.
For example, printing presses typically include a defect detection system as are known in the art. This type of defect detection system scans, and acquires an image of the printed web. The acquired image is subsequently compared to a stored digital template image. Any discrepancy between the acquired image and the template image beyond some tolerance is considered to be a defect. The isolated defects are then logged in a data file. When the systems detect a large change in color due to a change in inking level, a non-isolated defect is reported over a large portion of the web. When non-isolated defects are reported, an alarm will subsequently be set off to alert an operator to take appropriate corrective action.
Once a printed product is determined to be acceptable, the defect detection control systems will typically establish a new template image by scanning the acceptable printed product. The defect detection control system is not fully functional until a printed product is determined acceptable. While a template image can be collected before the printed product is considered acceptable, the template image may actually contain a defect, and an actual defective image may be considered acceptable or good, and therefore no corrective action is taken.
Furthermore, the printed product may have subtle defects even when it is judged acceptable. For example, a printing plate may have been scratched before the printing process started, or a blanket flaw such as a hickey or indentation may have been present.
The makeready process typically includes a visual comparison and inspection of a print product against a contract proof. This visual comparison and inspection process establishes that no formatting errors are introduced into the press between making the contract proof and putting the printing plates on press. However, typical defect detection control systems do not allow for a template image that has been collected based on a contract proof, or based on a digital representation of the printed work that was used to create the printing plate.
Traditionally, color control systems and defect detection control systems are two separate systems operating on a printing press. These separate systems utilize separate web scanning mechanisms. Image processing is often duplicated in these two control systems as well.